We have a lead free assembly with a 128-Pin PQFP with 20 mil pitch leads. We have tried stencil aperture reductions, step-down stencils, different thickness stencils, various printer setting changes, placement pressure changes, nozzle changes, moved the placement to a different machine, profile changes and still are having extreme difficulties with bridging. Most of the time, the bridging is not just between two leads but extends accross multiple leads. The paste job is consistent and looks very good as does the placement. Any less paste deposition and we'd have insufficient problems. If you look at a panel prior to reflow, you might see one placement that doesn't look as perfect as the rest and that might be the one that comes out the best - in other words, we don't see much consistency as to where the bridging might occur. We run the exact same product leaded and have very little trouble. Any ideas about what I should try next?

Bridging most contributed is from printing set up which is the parameter setting as well the board thickness setting. I would suggest you may check the parameter setting which mainly the squegee speed, stencil gap between PCB contact, printer machine humidity temperature cannot warm. I hope this would be help you.

Thanks all for the replies. Chunks has hit the nail on the head as the distance between pads is so little that the boardhouse was unable to apply soldermask. We are in the process of having the pads resized to allow for soldermask. Hopefully this is the key - I'll let you know when we get new boards in.

Yes - we got stuck in a train of thought that we have no troubles with the leaded product and should be able to do it lead-free. We overlooked what Chunks has pointed out and spent most of our time on process instead of looking at the boards closely. The lead free boards are a different part number than the leaded and might be a different supplier but I'm not sure about that...

When you say that the paste job "looks" good, are you actually taking a 3d measurement of the paste deposit, or are you doing it by sight? A good 3d paste measurement tool, in this case, is worth it's weight in gold. ASC International produces a line of 3d paste measurement tools that are very economical, and allow you to get a look at what is REALLY happening.